The present invention relates to a filtering device intended to eliminate a DC component from an electronic signal.
Such filtering devices are currently used in apparatus used for the reception of radio signals, such as television sets or radiotelephones. These apparatus conventionally comprise a receiving circuit, arranged at the end of an antenna and filter system intended to ensure the reception of a radio signal and to deliver, on an output, an electronic signal that represents said signal, which receiving circuit comprises an amplifier connected to the output of the antenna and filter system and intended to deliver an amplified electronic signal, a mixer having a first input connected to the output of the amplifier, a second input intended to receive a signal coming from a local oscillator, and an output, and a demodulator having an input connected to the output of the mixer and an output intended to deliver a frequency-demodulated audio signal.
The output of the mixer and the input of the demodulator are often twinned, intended to deliver and receive respectively pairs of signals having the same shape in phase quadrature. The elements included in the receiving circuit described above generally considerably amplify the signals they process, because of the low initial amplitude of the radio signal. If the signal processed by one of the elements mentioned above possesses a DC component, this component will also be amplified, thus causing a considerable shift of the AC component of the processed signal, which AC component contains the useful information transported by the signal. Such a shift may cause a saturation of output stages intended for the amplification of the processed signal, or also input stages of elements intended to receive the amplified signal and thus cause a peak limiting of said signal, leading to an alteration of the information it represents, which is unacceptable.
Accordingly, in the apparatus intended for the reception of radio signals described above, a filtering device is often inserted between the mixer and the demodulator with the aim of eliminating the DC component from the output signal of the mixer. The filtering devices used for this purpose are often high-pass filters formed by a resistor and a capacitor, which filters have a threshold frequency beyond which they pass signals, which frequency is chosen to be lower than the frequency of the AC component of the mixer output signal which is fixed and called intermediate frequency.
The use of such a high-pass filter raises considerable drawbacks. The value of the intermediate frequency being of the order of ten or one hundred kHz, for the threshold frequency one conventionally chooses a value of the order of one kHz for the cut-off frequency. As this value is relatively low, the realization of the high-pass filter requires a capacitor having a relatively high value, of the order of one NanoFarad. In the current state of integrated circuit technology, such a capacitor can only be realized in a discrete form, which implies that the use of a high-pass filter described above is an obstacle to the complete integration of the receiving circuit, which is, however, desirable in view of the reduction of the bulkiness and the consumption of said circuit. Besides, a high-pass filter including a high-value capacitor has a long time constant and thus introduces inside the receiving circuit considerable time-dependent inertia. In radiotelephones in accordance with the GSM or TDMA standards, the receiving circuit is only active during brief periods in which said circuit is to ensure the reception of a data packet, after which it is again deactivated. Each activation of the receiving circuit is followed by an initialization phase, after which the reception is possible. The time-dependent inertia caused by the high-value capacitor included in the high-pass filter may increase the duration of the initialization phase at the expense of the time interval allocated to the circuit for receiving the data packet, which may be harmful to the proper operation of the receiving circuit. Finally, the frequency demodulation of the output signal of the mixer must not be disturbed by a dissymmetry of the signal with regard to the intermediate frequency. Consequently, the receiving circuit is to provide an as much as possible symmetrical transfer characteristic around the intermediate frequency. The transfer characteristic of a high-pass filter is symmetrical around the zero frequency and thus causes a considerable dissymmetry of the transfer characteristic of the receiving circuit with regard to the intermediate frequency, which may also be undesirable for the proper operation of said circuit.
It is an object of the present invention to largely remedy these drawbacks by proposing an integrable filtering device which has a time-dependent inertia that is smaller than that of known filtering devices and preserves the symmetry of the transfer characteristic of the receiving circuit with regard to the intermediate frequency.
Indeed, a filtering device according to the invention, having a first and a second input to receive a first and a second input signal respectively, the second input signal being identical to the first but in phase quadrature to the first, and a first and a second output intended to deliver a first and a second output signal respectively, the second output signal being identical to the first but in phase quadrature to the first, comprises: a first elliptical filter including N capacitive elements Ci (for i=1 to N), arranged between the first input and the first output, the filter having a transfer characteristic that offers at least a zero at a predetermined frequency called the cut-off frequency, a second elliptical filter, identical to the first elliptical filter, arranged between the second input and the second output, and N gyrators Gi (for i=1 to N), each connected in parallel between two corresponding capacitive elements Ci (for i=1 to N), of the first and second elliptical filters, the gyrators being designed so that the filtering device has a similar transfer characteristic to that of the first and second elliptical filters while featuring a shift whose extent is equal to the absolute value of the cut-off frequency.
In such a filtering device, one may use easily integrable low-value capacitors by choosing a relatively high value for the cut-off frequency. The gyrators then provide the shift of the transfer characteristic of the filtering device. Besides, as will be seen hereinafter, this transfer characteristic regains a shape corresponding to a bandpass filter beyond the disturbance caused by the zero of the filter. The symmetry of the transfer characteristic of the filtering device according to the invention is thus largely preserved.
In a particular embodiment of the invention, a filtering device as described above is characterized in that, while each gyrator Gi (for i=1 to N) has a transconductance gi and has a first and a second input intended to receive a first and a second current I1i and I2i respectively, and a first and a second output intended to deliver the first and second current I1i and I2i respectively, and said gyrator is intended to develop a first voltage V1i between its first input and output, and a second voltage V2i between its second input and output, the behavior of each gyrator is defined by the relationships I1i=(gi)(V2i) and I2i=(xe2x88x92gi)(V1i), and gi=(2Txcfx80)(Fc)(Ci), where Fc is the absolute value of the cut-off frequency.
Such a configuration of the gyrators enables to easily obtain the shift of the zero of the transfer characteristic of the filtering device, so that this zero corresponds to the zero frequency.
In a preferred embodiment of the invention, all the capacitive elements Ci (for i=1 to N) included in the first and second elliptical filters have an identical nominal value, and all the gyrators Gi (for i=1 to N) are mutually identical.
The fact that the nominal values of the capacitors are the same and the fact that the gyrators are identical enables to reduce the influence of the imperfections due to the manufacturing process of the filtering device, said imperfections thus having the same effect on all the gyrators and all the capacitors in theory. The choice to have the same nominal values of the capacitive elements Ci thus enables to obtain a homogeneity of the dynamic of the signal flowing through the filtering device. Finally, such a choice enables to simplify and optimize the design and the floorplan of the integrated circuit including the filtering device, and thus to reduce its size and cost.
As explained above, the use of a filtering device in accordance with the invention is particularly advantageous inside a radio signal receiving circuit. Hence the invention also relates to a radiotelephony device, comprising an antenna and filter system intended to ensure the reception of a radio signal and to deliver on an output an electronic signal representing said signal, an amplifier connected to the output of the antenna and filter system and intended to deliver an amplified electronic signal, a mixer having a first input connected to the output of the amplifier, a second input intended to receive a signal that comes from a local oscillator, and an output intended to deliver a signal that has a fixed frequency called intermediate frequency, and, a demodulator having an input connected to the output of the mixer and an output intended to deliver a frequency-demodulated audio signal, which device is characterized in that it further includes a filtering device as described above, inserted between the mixer and the demodulator, while the elliptical filters included in said filtering device have the intermediate frequency for their cut-off frequency.
Finally, in the most general form, the invention relates to a method of eliminating a DC component from an electronic signal, comprising making a configuration of a filter so that it has a transfer characteristic that offers at least a zero at a predetermined frequency, called the cut-off frequency, providing a shift of said transfer characteristic, the extent of which shift is equal to the absolute value of the cut-off frequency, and causing said electronic signal to flow through said filter thus obtained.
These and other aspects of the invention are apparent from and will be elucidated, by way of non-limitative example, with reference to the embodiment(s) described hereinafter.